Interbody fusion device and method for restoration of normal spinal anatomy

ABSTRACT

An interbody fusion device in one embodiment includes a tapered body defining a hollow interior for receiving bone graft or bone substitute material, The body defines exterior threads which are interrupted over portions of the outer surface of the device. The fusion device defines truncated side walls so that on end view the body takes on a cylindrical form. The side walls are provided with vascularization openings, and the body wall device includes opposite bone ingrowth slots extending through the interrupted thread portion of the body. In another embodiment, the tapered body is solid and formed of a porous biocompatible material having sufficient structural integrity to maintain the intradiscal space and normal curvature. The material is preferably a porous tantalum having fully interconnected pores to facilitate complete bone tissue ingrowth into the implant. An implant driver is provided which engages the truncated side walls to complete the cylindrical form of the implant at the root diameter of the interrupted threads, to thereby facilitate threaded insertion of the implant to the intra-discal space between adjacent vertebrae. Methods for posterior and anterior insertion of the fusion device are also disclosed.

BACKGROUND OF THE INVENTION

The present application is a continuation-in-part of co-pendingapplication, Ser. No. 08/411,017, having the same title and namedinventors, filed on Mar. 27, 1995, and still pending.

The present invention relates to an artificial implant to be placed intothe intervertebral space left after the removal of a damaged spinaldisc. Specifically, the invention concerns an implant that facilitatesarthrodesis or fusion between adjacent vertebrae while also maintainingor restoring the normal spinal anatomy at the particular vertebrallevel.

The number of spinal surgeries to correct the causes of low back painhas steadily increased over the last several years. Most often, low backpain originates from damage or defects in the spinal disc betweenadjacent vertebrae. The disc can be herniated or can be suffering from avariety of degenerative conditions, so that in either case theanatomical function of the spinal disc is disrupted. The most prevalentsurgical treatment for these types of conditions has been to fuse thetwo vertebrae surrounding the affected disc. In most cases, the entiredisc will be removed, except for the annulus, by way of a discectomyprocedure. Since the damaged disc material has been removed, somethingmust be positioned within the intra-discal space, otherwise the spacemay collapse resulting in damage to the nerves extending along thespinal column.

In order to prevent this disc space collapse, the intra-discal space isfilled with bone or a bone substitute in order to fuse the two adjacentvertebrae together. In early techniques, bone material was simplydisposed between the adjacent vertebrae, typically at the posterioraspect of the vertebrae, and the spinal column was stabilized by way ofa plate or a rod spanning the affected vertebrae. With this techniqueonce fusion occurred the hardware used to maintain the stability of thesegment became superfluous. Moreover, the surgical procedures necessaryto implant a rod or plate to stabilize the level during fusion werefrequently lengthy and involved.

It was therefore determined that a more optimum solution to thestabilization of an excised disc space is to fuse the vertebrae betweentheir respective end plates, most optimally without the need foranterior or posterior plating. There have been an extensive number ofattempts to develop an acceptable intra-discal implant that could beused to replace a damaged disc and yet maintain the stability of thedisc interspace between the adjacent vertebrae, at least until completearthrodesis is achieved. These "interbody fusion devices" have takenmany forms. For example, one of the more prevalent designs takes theform of a cylindrical implant. These types of implants are representedby the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat.No. 4,878,915; Ray, U.S. Pat. Nos. 4,961,740 and 5,055,104; andMichelson, U.S. Pat. No. 5,015,247. In these cylindrical implants, theexterior portion of the cylinder can be threaded to facilitate insertionof the interbody fusion device, as represented by the Ray, Brantigan andMichelson patents. In the alternative, some of the fusion implants aredesigned to be pounded into the intra-discal space and the vertebral endplates. These types of devices are represented by the patents toBrantigan, U.S. Pat. Nos. 4,743,256; 4,834,757 and 5,192,327.

In each of the above listed patents, the transverse cross section of theimplant is constant throughout its length and is typically in the formof a right circular cylinder. Other implants have been developed forinterbody fusion that do not have a constant cross section. Forinstance, the patent to McKenna, U.S. Pat. No. 4,714,469 shows ahemispherical implant with elongated protuberances that project into thevertebral end plate. The patent to Kuntz, U.S. Pat. No. 4,714,469, showsa bullet shaped prosthesis configured to optimize a friction fit betweenthe prosthesis and the adjacent vertebral bodies. Finally, the implantof Bagby, U.S. Pat. No. 4,936,848 is in the form of a sphere which ispreferably positioned between the centrums of the adjacent vertebrae.

Interbody fusion devices can be generally divided into two basiccategories, namely solid implants and implants that are designed topermit bone ingrowth. Solid implants are represented by U.S. Pat. Nos.4,878,915; 4,743,256; 4,349,921 and 4,714,469. The remaining patentsdiscussed above include some aspect that permits bone to grow across theimplant. It has been found that devices that promote natural boneingrowth achieve a more rapid and stable arthrodesis. The devicedepicted in the Michelson patent is representative of this type ofhollow implant which is typically filled with autologous bone prior toinsertion into the intra-discal space. This implant includes a pluralityof circular apertures which communicate with the hollow interior of theimplant, thereby providing a path for tissue growth between thevertebral end plates and the bone or bone substitute within the implant.In preparing the intra-discal space, the end plates are preferablyreduced to bleeding bone to facilitate this tissue ingrowth. Duringfusion, the metal structure provided by the Michelson implant helpsmaintain the patency and stability of the motion segment to be fused. Inaddition, once arthrodesis occurs, the implant itself serves as a sortof anchor for the solid bony mass.

A number of difficulties still remain with the many interbody fusiondevices currently available. While it is recognized that hollow implantsthat permit bone ingrowth into bone or bone substitute within theimplant is an optimum technique for achieving fusion, most of the priorart devices have difficulty in achieving this fusion, at least withoutthe aid of some additional stabilizing device, such as a rod or plate.Moreover, some of these devices are not structurally strong enough tosupport the heavy loads and bending moments applied at the mostfrequently fused vertebral levels, namely those in the lower lumbarspine.

There has been a need for providing an interbody fusion device thatoptimizes the bone ingrowth capabilities but is still strong enough tosupport the spine segment until arthrodesis occurs. It has been found bythe present inventors that openings into a hollow implant for boneingrowth play an important role in avoiding stress shielding of theautologous bone impacted within the implant. In other words, if theingrowth openings are improperly sized or configured, the autologousbone will not endure the loading that is typically found to be necessaryto ensure rapid and complete fusion. In this instance, the bone impactedwithin the implant may resorb or evolve into simply fibrous tissue,rather than a bony fusion mass, which leads to a generally unstableconstruction. On the other hand, the bone ingrowth openings must not beso extensive that the cage provides insufficient support to avoidsubsidence into the adjacent vertebrae.

Another problem that is not addressed by the above prior devicesconcerns maintaining or restoring the normal anatomy of the fused spinalsegment. Naturally, once the disc is removed, the normal lordotic orkyphotic curvature of the spine is eliminated. With the prior devices,the need to restore this curvature is neglected. For example, in onetype of commercial device, the BAK device of SpineTech, as representedby the patent to Bagby, U.S. Pat. No. 4,501,269, the adjacent vertebralbodies are reamed with a cylindrical reamer that fits the particularimplant. In some cases, the normal curvature is established prior toreaming and then the implant inserted. This type of construct isillustrated in FIG. I which reveals the depth of penetration of thecylindrical implant into the generally healthy vertebrae adjacent theinstrumented discal space. However, this over-reaming of the posteriorportion is generally not well accepted because of the removal of loadbearing bone of the vertebrae, and because it is typically difficult toream through the posterior portion of the lower lumbar segment where thelordosis is greatest. In most cases using implants of this type, noeffort is made to restore the lordotic curvature, so that thecylindrical implant is likely to cause a kyphotic deformity as thevertebra settles around the implant. This phenomenon can often lead torevision surgeries because the spine becomes imbalanced.

In view of these limitations of the prior devices, there remains a needfor an interbody fusion device that can optimize bone ingrowth whilestill maintaining its strength and stability. There is further a needfor such an implant that is capable of maintaining or restoring thenormal spinal anatomy at the instrumented segment. This implant must bestrong enough to support and withstand the heavy loads generated on thespine at the instrumented level, while remaining stable throughout theduration.

SUMMARY OF THE INVENTION

In response to the needs still left unresolved by the prior devices, thepresent invention contemplates a hollow threaded interbody fusion deviceconfigured to restore the normal angular relation between adjacentvertebrae. In particular, the device includes an elongated body, taperedalong substantially its entire length, defining a hollow interior andhaving an outer diameter greater than the size of the space between theadjacent vertebrae. The body includes an outer surface with oppositetapered cylindrical portions and a pair of opposite flat tapered sidesurfaces between the cylindrical portions. Thus, at an end view, thefusion device gives the appearance of a cylindrical body in which thesides of the body have been truncated along a chord of the body's outerdiameter. The cylindrical portions are threaded for controlled insertionand engagement into the end plates of the adjacent vertebrae.

In another aspect of the invention, the outer surface is tapered alongits length at an angle corresponding, in one embodiment, to the normallordotic angle of lower lumbar vertebrae. The outer surface is alsoprovided with a number of vascularization openings defined in the flatside surfaces, and a pair of elongated opposite bone ingrowth slotsdefined in the cylindrical portions. The bone ingrowth slots have atransverse width that is preferably about half of the effective width ofthe cylindrical portions within which the slots are defined.

In another embodiment, the interbody fusion device retains the sametapered configuration of the above embodiment, along with the truncatedsidew walls and interrupted external threads. However, in thisembodiment, the implant is not hollow but is instead solid. Boneingrowth is achieved by forming the solid tapered implant of a poroushigh strength material tht permits bone ingrowth into interconnectedpores while retaining sufficient material for structural stability insitu. In one preferred embodiment, the material is porous tantalum.

A driving tool is provided for inserting the fusion device within theintra-discal space. In one feature, the driving tool includes a shafthaving a pair of opposite tapered tongs situated at one end. The tongsare connected to the shaft by way of a hinge slot that biases the tongsapart to receive a fusion device therebetween. The driving tool isfurther provided with a sleeve concentrically disposed about the shaftand configured to slide along the shaft and compress the hinge to pushthe tongs together to grip the fusion device. Alternatively, an internalexpanding collet may be used to internally hold the fusion devicesecurely during insertion.

In one aspect of the driving tool, the tapered tongs have an outersurface that takes on the form of the tapered cylindrical portions ofthe fusion device. The tongs also have a flat inward facing surface tocorrespond to the flat side surfaces of the fusion device. Thus, whenthe tongs are compressed against the fusion device, the inward facingsurfaces of the tongs contact the flat sides of the fusion device andthe outer surface of the tongs complete the conical form of the fusiondevice to facilitate screw-in insertion. The inward facing surface ofthe tongs may also be provided with projections to engage openings inthe fusion device to permit driving and rotation of the device withinthe intra-discal space.

In another aspect of the invention, methods are provided for implantingthe fusion device between adjacent vertebrae. In one method, theapproach is anterior and includes the steps of dilating the disc spaceand drilling the end plates of the adjacent vertebrae to the minordiameter of the fusion device threads. A sleeve is inserted to provide aworking channel for the drilling step and the subsequent step ofimplanting the fusion device. The implant is engaged with the drivingtool, inserted through the sleeve and threaded into the prepared bore.The depth of insertion of the tapered fusion device determines theamount of angular separation achieved for the adjacent vertebrae.

In another inventive method, the insertion site is prepared posteriorly,namely the disc space is dilated and a minor diameter hole is drilledinto the vertebral end plates. A sleeve is also arranged to provide aworking channel for the drilling and insertion steps. The fusion deviceis inserted into the drilled hole with the flat side walls facing theadjacent vertebra. The device is then rotated so that the externalthreads on the cylindrical portion cut into and engage the adjacentvertebrae. In addition, since the fusion device is tapered, the taperedouter surface of the device will angularly separate the adjacentvertebrae to restore the normal anatomic lordosis.

DESCRIPTION OF THE FIGURES

FIG. 1 is a side-elevational view in the sagittal plane of a fusiondevice of the prior art.

FIG. 2 is an enlarged perspective view of an interbody fusion deviceaccording to one embodiment of the present invention.

FIG. 3 is a side cross-sectional view of the interbody fusion deviceshown in FIG. 2, taken along line 3--3 as viewed in the direction of thearrows.

FIG. 4 is an end elevational view from the anterior end of the interbodyfusion device shown in FIG. 2.

FIG. 5 is a top-elevational view of the interbody fusion device shown inFIG. 2.

FIG. 6 is an A-P lateral view from the anterior aspect of the spineshowing two interbody fusion devices according to FIG. 2 implantedwithin the interbody space between L4 and L5.

FIG. 7 is a sagittal plane view of the interbody fusion device implantedbetween L4 and L5 shown in FIG. 6.

FIG. 8 is a perspective view of an alternative embodiment of theinterbody fusion device according to the present invention.

FIG. 8A is a perspective view of another embodiment of a taperedinterbody fusion device according to the present invention.

FIG. 9 is a top-elevational view of an implant driver according toanother aspect of the present invention.

FIG. 10 is an enlarged perspective view of the end of the implant driverengaged about an interbody fusion device, as depicted in FIG. 2.

FIG. 11 is an enlarged partial side cross-sectional view showing theimplant driver engaging the interbody fusion device, as shown in FIG.10.

FIG. 12 is an enlarged partial side cross-sectional view showing animplant driver of an alternative embodiment adapted for engaging theinterbody fusion device 10.

FIGS. 13(a)-12(d) show four steps of a method in accordance with oneaspect of the invention for implanting the interbody fusion device, suchas the device shown in FIG. 2.

FIGS. 14(a)-13(d) depict steps of an alternative method for implantingthe interbody fusion device, such as the device shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

An interbody fusion device 10 in accordance with one aspect of thepresent invention is shown in FIGS. 2-5. The device is formed by a solidconical body 11, that is preferably formed of a biocompatible or inertmaterial. For example, the body 11 can be made of a medical gradestainless steel or titanium, or other suitable material having adequatestrength characteristics set forth herein. The device may also becomposed of a biocompatible porous material, such as porous tantalumprovided by Implex Corp. For purposes of reference, the device 10 has ananterior end 12 and a posterior end 13, which correspond to the anatomicposition of the device 10 when implanted in the intra-discal space. Theconical body 11 defines a hollow interior 15 which is bounded by a bodywall 16 and closed at the posterior end 13 by an end wall 17 (see FIG.3). The hollow interior 15 of the device 10 is configured to receiveautograft bone or a bone substitute material adapted to promote a solidfusion between adjacent vertebrae and across the intra-discal space.

In accordance with the invention, the interbody fusion device 10 is athreaded device configured to be screw threaded into the end plates ofthe adjacent vertebrae. In one embodiment of the invention, the conicalbody 11 defines a series of interrupted external threads 18 and acomplete thread 19 at the leading end of the implant. The completethread 19 serves as a "starter" thread for screwing the implant into thevertebral endplates at the intra-discal space. The threads 18 and 19 cantake several forms known in the art for engagement into vertebral bone.For instance, the threads can have a triangular cross-section or atruncated triangular cross-section. Preferably, the threads have aheight of 1.0 mm (0.039 in) in order to provide adequate purchase in thevertebral bone so that the fusion device 10 is not driven out of theintra-discal space by the high loads experienced by the spine. Thethread pitch in certain specific embodiments can be 2.3 mm (0.091 in) or3.0 mm (0.118 in), depending upon the vertebral level at which thedevice 10 is to be implanted and the amount of thread engagementnecessary to hold the implant in position.

In one aspect of the invention, the conical body 11, and particularlythe body wall 16, includes parallel truncated side walls 22, shown mostclearly in FIG. 4. The side walls are preferably flat to facilitateinsertion of the fusion device between the end plates of adjacentvertebrae and provide area between for bony fusion. The truncated sidewalls extend from the anterior end 12 of the device up to the completethreads 19 at the posterior end 13. Thus, with the truncated side walls22, the device 10 gives the appearance at its end view of an incompletecircle in which the sides are cut across a chord of the circle. In onespecific example, the interbody fusion device 10 has a diameter at itsanterior end of 16.0 mm (0.630 in). In this specific embodiment, thetruncated side walls 22 are formed along parallel chord linesapproximately 12.0 mm (0.472 in) apart, so that the removed arc portionof the circle roughly subtends 90° at each side of the device. Otherbenefits and advantages provided by the truncated side walls 22 of thefusion device 10 will be described in more detail herein.

The conical body 11 of the device 10 includes a pair of vascularizationopenings 24 and 25 defined through each of the truncated side walls 22.These openings 24 and 25 are adapted to be oriented in a lateraldirection or facing the sagittal plane when the fusion device isimplanted within the intra-discal space. The openings are intended toprovide a passageway for vascularization to occur between the boneimplant material within the hollow interior 15 and the surroundingtissue. In addition, some bone ingrowth may also occur through theseopenings. The openings 24 and 25 have been sized to provide optimumpassage for vascularization to occur, while still retaining asignificant amount of structure in the conical body 11 to support thehigh axial loads passing across the intra-discal space between adjacentvertebrae.

The conical body 11 also defines opposite bone ingrowth slots 27, eachof which are oriented at 90° to the truncated side walls 22. Preferably,these slots 27 are directly adjacent the vertebral end plates when thedevice 10 is implanted. More particularly, as the threads 18 and 19 ofthe device are screwed into the vertebral endplates, the vertebral bonewill extend partially into the slots 27 to contact bone implant materialcontained within the hollow interior 15 of the device 10. As shown moreclearly in FIG. 5, the bone ingrowth slots 27 are configured to providemaximum opening for bone ingrowth, in order to ensure completearthrodesis and a solid fusion. Preferably, the slots have a lateralwidth that approximates the effective width of the threaded portions ofthe body. It has been found that the prior devices which utilize aplurality of small apertures do not promote a rapid and solidarthrodesis of the bone material. Instead, the smaller apertures oftenlead to pseudo-arthrosis and the generation of fibrous tissue. Since thebone ingrowth slots 27 of the present invention are directly facing thevertebrae, they are not situated in a portion of the device that mustbear high loads. Instead, the truncated side walls 22 will bear most ofthe load passing between the vertebral end plates through theinterrupted threads 18 and across the intra-discal space.

In a further feature, the anterior end 12 of the body wall 16 can definea pair of diametrically opposed notches 29, which are configured toengage an implant driver tool as described herein. Moreover, the endwall 17 at the posterior end 13 of the implant can be provided with atool engagement feature (not shown). For example, a hex recess can beprovided to accommodate a hex driver tool, as described further herein.

In one important feature of the interbody fusion device of the presentinvention, the body 11 includes a tapered or conical form. In otherwords, the outer diameter of the device at its anterior end 12 is largerthan the outer diameter at the posterior end 13. As depicted in FIG. 3,the body wall 16 tapers at an angle A about the centerline CL of thedevice 10. The taper of the body wall 16 is adapted to restore thenormal relative angle between adjacent vertebrae. For example, in thelumbar region, the angle A is adapted to restore the normal lordoticangle and curvature of the spine in that region. In one specificexample, the angle A is 8.794°. It is understood that the implant mayhave non-tapered portions, provided that the portions do not otherwiseinterfere with the function of the tapered body.

The taper angle A of the implant, coupled with the outer diameter at theanterior and posterior ends of the fusion device 10, define the amountof angular spreading that will occur between the adjacent vertebrae asthe implant is placed or screwed into position. This feature is depictedmore clearly in FIGS. 6 and 7 in which a preferred construct employing apair of fusion devices 10 is shown. In the depicted construct, thedevices 10 are disposed between the lower lumbar vertebrae L4 and L5,with the threads 18 and 19 threaded into the end plates E of the twovertebrae. As shown in FIG. 7, as the device 10 is threaded into the endplates E, it advances in the direction of the arrow I toward the pivotaxis P of the vertebral level. The pivot axis P is nominally the centerof relative rotation between the adjacent vertebrae of the motionsegment. As the tapered fusion device 10 is driven further in thedirection of the arrow I toward the pivot axis P, the adjacent vertebraeL4 and L5 are angularly spread in the direction of the arrows S. Depthof insertion of the fusion device 10 will determine the ultimatelordotic angle L achieved between the two vertebrae.

In specific embodiments of the implant 10, the outer diameter or threadcrest diameter at the anterior end 12 can be 16, 18 or 20 mm, and theoverall length of the device 26 mm. The sizing of the device is drivenby the vertebral level into which the device is implanted and the amountof angle that must be developed.

In another aspect of the invention, device 10 is sized so that two suchcylindrical bodies 11 can be implanted into a single disc space, asshown in FIG. 6. This permits the placement of additional bone graftmaterial between and around the devices 10 in situ. This aspect furtherpromotes fusion across the intra-discal space and also serves to morefirmly anchor the devices between the adjacent vertebrae to preventexpulsion due to the high axial loads at the particular vertebral level.

In one specific embodiment of the interbody fusion device 10, thevascularization opening 24 is generally rectangular in shape havingdimensions of 6.0 mm (0.236 in) by 7.0 mm (0.276 in). Similarly, thevascularization opening 25 is rectangular with dimensions of 4.0 mm(0.157 in) by 5.0 mm (0197 in). Naturally, this opening is smallerbecause it is disposed at the smaller posterior end 13 of the device 10.The bone ingrowth slots 27 are also rectangular in shape with a longdimension of 20.0 mm (0.787 in) and a width of 6.0 mm (0.236 in). It hasbeen found that these dimensions of the vascularization openings 24, 25and slots 27 provide optimum bone ingrowth and vascularization. Inaddition, these openings are not so large that they compromise thestructural integrity of the device or that they permit the bone graftmaterial contained within the hollow interior 15 to be easily expelledduring implantation.

As can be seen in FIG. 7, when the device is in position between the L4and L5 vertebrae, the vascularization openings 24 and 25 are side facingto contact the highly vascularized tissue surrounding the vertebrae. Inaddition, as can be seen in FIG. 6, the bone ingrowth slots 27 areaxially directed so that they contact the vertebral end plates E.

In an alternative embodiment of the invention, shown in FIG. 8, aninterbody fusion device 30 is formed of a conical body 31. The body wall34 defines a hollow interior 33 as with the fusion device 10 of theprevious embodiment. However, in this embodiment the truncated side wall38 does not include any vascularization openings. Moreover, the boneingrowth slots 39 on opposite sides of the device 30 are smaller. Thismeans that the interrupted threads 36 on the exterior of the device 30extend a greater length around the implant. Such a design could beutilized if a porous material (e.g., porous tantalum) were used toprovide additional surface area for tissue ingrowth and anchorage to theadjacent bone. Also, this interbody fusion device 30 of the embodimentshown in FIG. 8 can have application at certain vertebral levels wherethe risk of expulsion of the device is greatest. Consequently, theamount of thread contact is increased to prevent such expulsion. Priorto insertion, the hollow interior 15 of the fusion device 10 is filledcompletely with bone or substitute to facilitate this pre-loading.

In a further embodiment using a porous material, the interbody fusiondevice 110 of FIG. 8A retains the tapered configuration of the previousembodiments, but is solid instead of hollow. The device 110 comprises atapered body 111 having an larger outer diameter at its anterior end 112than at its posterior end 113. The entire body 111 is solid leaving aclosed surface, such as surface 115, at both ends of the implant. Thedevice includes the interrupted threads 118, starter threads 119 andtruncated side walls 122 of the prior embodiments. A driving tool slot129 can also be defined in the end surface 115. Alternatively, thestarter threads 119 can be eliminated leaving an unthreaded cylindricalportion at the posterior end of the implant. Similarly, the driving toolslot 129 take on many configurations depending upon the design of thetool used to insert the device 110 into the intradiscal space.

The benefits of the embodiment of the fusion device shown in FIG. 8A areespecially appreciated by the use of a porous, high strength material toform the solid body 111. In the preferred embodiment, this material is aporous tantalum-carbon composite marketed by Implex Corp. under thetradename HEDROCEL® and described in U.S. Pat. No. 5,282,861 to Kaplan,which description is incorporated herein by reference. Due to the natureof the HEDROCEL® material, the entire exterior surface of the solid body111 includes pores 130 that are interconnected throughout the body. Thesubstrate of the HEDROCEL® carbon-tantalum composite is a skeleton ofvitreous carbon, or a reticulated open cell carbon foam, which defines anetwork of interconnecting pores. The substrate is infiltrated with avapor-deposited thin film of tantalum.

HEDROCEL® is preferred because it provides the advantages of both metaland ceramic implants without the corresponding disadvantages. HEDROCELis well suited for the interbody fusion device of the present inventionbecause it mimics the structure of bone and has a modulus of elasticitythat approximates that of human bone. The interconnected porosityencourages bone ingrowth and eliminates dead ends which limitvascularization of the bone. The infiltrated metal film providesstrength and stiffness without significant weight increase. A HEDROCEL®implant is sufficiently strong to maintain the intervertebral space andnormal curvature of the spine at the instrumented motion segment. At thesame time, stress shielding is avoided. This composite material is alsoadvantageous because it eliminates the need for allografts orautografts.

One additional advantage of this material is that it does not undergoresorption. This prevents early degradation which can inhibit boneregeneration. A non-resorbable implant is also beneficial where completebone ingrowth may not be achieved. Disadvantages of permanent,nonresorbable implants, however, are avoided because of the excellentbiocompatibility and osteoconductivity of the composite.

While HEDROCEL® is preferred, it is contemplated that any suitable highstrength porous material may be used. Other open-celled substrates andmetals are contemplated. For example, the substrate may be othercarbonaceous materials, such as graphite, or ceramics, such astricalcium phosphate or calcium aluminate. Any suitable metal iscontemplated, but Group VB elements, such as tantalum and niobium, andtheir alloys, are preferred. Tantalum is particularly preferred for itsgood mechanical properties and biocompatibility.

The interbody fusion device 10 can be implanted using an implant driver50, shown in FIG. 9, according to one aspect of the invention. Theimplant driver 50 is comprised of a shaft 51 and sleeve 52concentrically disposed about the shaft. Tongs 54 are formed at one endof the shaft for gripping the interbody fusion device 10 forimplantation. The tongs include a tapered outer surface 55 and anopposite flat inner surface 56 adapted to engage the truncated sidewalls 22 of the interbody fusion device. The tapered outer surface 55conforms to the root diameter of the interrupted threads 18 so that thetongs 54 essentially complete the full cylindrical shape of the bodywall 16. The adaptation of the tong's tapered outer surface 55facilitates screw insertion of the interbody fusion device 10 since theouter surface 55 will ride within the tapped bore in the vertebralendplates.

Each of the tongs is provided with interlocking fingers 58 and a drivingprojection 59 extending from the inner surface 56. The function of thesecomponents is shown more clearly with reference to FIG. 11. Referringfirst to FIG. 9, the shaft 51 defines a hinge slot 62 supporting each ofthe pair of tongs 54. The hinge slot 62 is configured so that the tongswill have a naturally biased position spread sufficiently apart toaccept the tapered interbody fusion device 10 therebetween. The shaft 51defines a conical taper 63 between the hinged slot 62 and each of thetongs 54. This conical taper mates with a conical chamfer 67 defined onthe inner wall of the sleeve 52. Thus, as the sleeve 52 is advancedtoward the tongs 54, the conical chamfer 67 rides against the conicaltaper 63 to close or compress the hinge slot 62. In this manner, thetongs 54 are pushed toward each other and pressed into grippingengagement with the interbody fusion device situated between the tongs.

The shaft 51 and sleeve 52 are provided with a threaded interface 65which permits the sleeve 52 to be threaded up and down the length theshaft. Specifically, the threaded interface 65 includes external threadson the shaft 51 and internal threads on the sleeve 52 having the samepitch so that the sleeve can be readily moved up and down the implantdriver 50. The shaft 51 is also provided with a pair of stops 69 whichrestrict the backward movement of the sleeve 52 to only the extentnecessary to allow the tongs 54 to separate a sufficient distance toaccept the interbody fusion device 10.

The use of the implant driver 50 is shown with reference to FIGS. 10 and11. As can be seen in FIG. 10, the outer surface 55 of the tongs 54reside generally flush with the root diameter of the interrupted threads18. As seen in FIG. 11, the interlocking fingers 58 can be arranged tofit within the vascularization opening 24 on each of the truncated sidewalls 22. In a similar fashion, the driving projections 59 engage thedriving tool slots 29 at the anterior end 12 of the conical body 11. Thecombination of the interlocking fingers 58 and driving projections 59firmly engage the interbody fusion device 10 so that the device can bescrew threaded into a tapped or untapped opening in the vertebral bone.

An alternative embodiment of the implant driver is shown in FIG. 12. Thedriver 90 includes a shaft 91, having a length sufficient to reach intothe intradiscal space from outside the patient. Connected to the end ofshaft 91 is a head which defines a pair of opposite tongs 93, each ofwhich are configured for flush contact with the flat truncated sidewalls 22 of the fusion device 10. Like the tongs 54 of the previouslydescribed implant driver 50, the outer surface of the tongs iscylindrical to correspond to the cylindrical threaded portion of thedevice.

Unlike the implant driver 50, the driver 90 of the embodiment in FIG. 12uses an expanding collet assembly to firmly grip the fusion device 10for insertion into the body. Specifically, the head 92 defines a collet94 having a central collet bore 95 formed therethrough. The collet 94terminates in an annular flange 96 that at least initially has adiameter slightly smaller than the inner diameter of the fusion device10 at its end 12. An expander shaft 97 slidably extends through thecollet bore and includes a flared tip 98 situated adjacent and extendingjust beyond the annular flange 96. The flared tip 98 of the expandershaft 97 starts at a diameter sized to slide within the collet bore 95and gradually flares to a diameter larger than the bore.

The implant driver 90 includes a puller shaft 99 slidably disposedwithin a bore 100 defined in the shaft 91. The puller shaft 99 has alocking chamber 101 at its end which engages a locking hub 102 formed atthe end of the expander shaft 97. The puller shaft 99 projects beyondthe end of shaft 91 for access by the surgeon. When the puller shaft 99is pulled, it pulls the expander shaft 97 away from the annular flange96 of the collet 94 so that the flared tip 98 becomes progressivelyengaged within the collet bore 95. As the tip 98 advances further intothe bore 95, the annular flange 96 expands from its initial diameter toa larger second diameter sufficient for firm gripping contact with theinterior of the fusion device 10. With the fusion device so engaged, theimplant driver can be used to insert the device 10 into the surgicalsite, after which the expander shaft can be advanced beyond the colletbore to release the flared tip and, consequently, the fusion device.

In accordance with the present invention, two methods for implanting theinterbody fusion device 10 are contemplated. First, with reference toFIGS. 12(a)-12(d), an anterior approach is shown. As a preliminary step,it is necessary to locate appropriate starting points for implanting thefusion device, preferably bilaterally. In the first step of the anteriorapproach, a dilator 75 is disposed between the vertebral end plates E todilate the disc space between the L4 and L5 vertebrae. (It isunderstood, of course, that this procedure can be applied at othervertebral levels). In the second step, shown in FIG. 12(b), an outersleeve 76 is disposed about the disc space. The outer sleeve 76 can beof a known design that is configured to positively engage the anterioraspect of the vertebral bodies to firmly, but temporarily, anchor theouter sleeve 76 in position. In essence, this outer sleeve 76 operatesas a working channel for this laproscopic-type approach. In this step ofFIG. 12(b), a drill 77 of known design is extended through the outersleeve and used to drill out circular openings in the adjacent vertebralbodies. The openings can be tapped to facilitate screw insertion of thefusion device, although this step is not necessary.

In the next step shown in FIG. 12(c), the fusion device 10 is engaged bythe implant driver 50 and extended through the outer sleeve 76 until thestarter thread 19 contacts the bone opening. The implant driver 50 canthen be used to screw thread the fusion device into the tapped oruntapped opening formed in the vertebral end plate E. It is understoodthat in this step, other suitable driving tools could be used, such as ascrew driver type device to engage the driving tool slots 29 at theanterior end 12 of the device 10. As discussed previously, the degree ofinsertion of the fusion device 10 determines the amount of lordosisadded or restored to the vertebral level. In the final step, the implantdriver is removed leaving the fusion device 10 in position. It can beseen that once implanted, the closed end wall 17 is directed toward theposterior aspect of the vertebrae. The hollow interior 15 is open at itsanterior end, but can be closed by a plastic or metal material, ifnecessary.

In a second inventive method, as depicted in FIGS. 13(a)-13(d), aposterior approach is implemented. The first two steps of the posteriorapproach are similar to that of the prior anterior approach, except thatthe dilator 75, outer sleeve 76 and drill 77 are introduced posteriorlyinto the instrumented region. This approach may require decorticationand removal of vertebral bone to accept the outer sleeve 76. In thethird step of this method, the fusion device 10 is inserted through theouter sleeve 76 into the dilated disc space. It is understood that thedisc space is dilated only to the extent necessary to receive theimplant with the truncated side walls 22 directly facing the vertebralend plates E. Thus, as shown in FIG. 13(c), the bone ingrowth slot 27 isfacing laterally, rather than coronally, as expected for its finalimplanted position. A suitable driving tool 80 can be provided toproject the fusion device 10 through the outer sleeve 76 and into theintra-discal space. In one embodiment, the driving tool 80 includes aprojection 81 which is configured to engage a slot opening formed in theend wall 17 at the posterior end 13 of the fusion device 10. An internalthread (not shown) can be used to fix the device 10 to the driver 80.

Once the fusion device 10 has been advanced into the intra-discal spaceto the appropriate depth relative to the pivot axis P of the vertebrae,the driving tool 80 is used to rotate the implant in the direction ofthe rotational arrow R in FIG. 13(c). As the driving tool 80 is rotated,the device itself rotates so that the interrupted threads 18 startcutting into the vertebral bone at the end plates E. In this manner, theimplant operates as a cam to separate the adjacent vertebrae in thedirection of the spreading direction arrows S in FIG. 13(C). Thiscamming approach provides a somewhat easier insertion procedure in thata single rotation is required to lock the implant into the vertebralbone. In contrast, the formerly discussed screw insertion techniquerequires continuous threading of the device into position.

With either technique, the position of the fusion device 10 with respectto the adjacent vertebrae can be verified by radiograph or othersuitable techniques for establishing the angular relationship betweenthe vertebrae. Alternatively, the preferred depth of insertion of theimplant can be determined in advance and measured from outside thepatient as the implant is positioned between the vertebrae.

It can be seen that the interbody fusion device 10, implant driver 50and techniques of the present invention provide significant advantagesover the prior devices and techniques. Specifically, the fusion device10 provides a hollow threaded implant that maximizes the potential forbony fusion between adjacent vertebrae, while maintaining the integrityof the implant itself. It is understood that the spine enduressignificant loads along its axial length, which loads must be supportedby the fusion device 10 at least until solid fusion is achieved. Thedevice 10 also provides means for vascularization and tissue ingrowth tooccur which speeds up the fusion rate and enhances the strength of theresulting fused bony mass. Another significant aspect is that thetapered shape of the implant allows the surgeon to restore and maintainthe proper curvature or relative angle between vertebral bodies. Thisavoids the significant problems associated with prior devices in whichproduct deformities arise and the spine goes out of balance. A furtheradvantage achieved by the device and its implant driver is thecapability for insertion either anteriorly or posteriorly using alaproscopic approach. Depending upon the vertebral level, eitherapproach may be preferred, so it is important that the implant beadapted for insertion from either direction. Controlled insertion of thedevice is provided by the screw-in technique used for anterior insertion(vs. pounding in) and for the slide-in and cam method used for theposterior technique.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected. For example, while the device 10 has beendisclosed for use in the spine, the structure and procedures of thepresent invention can also be used in other joint spaces, such as theankle, wrist and subtalar joints. Moreover, while the device 10 of thepreferred embodiment is shown tapered along its entire length, it iscontemplated that a non-tapered or reverse tapered section can be addedwith the resulting device still falling within the scope of theinvention.

What is claimed is:
 1. A fusion device for facilitating arthrodesis inthe disc space between adjacent vertebrae, comprising:an elongated bodyhaving a length, a first diameter at a first end and a larger seconddiameter at a second end opposite said first end, said first and seconddiameters sized to be greater than the space between the adjacentvertebrae; said body having an outer surface that is substantiallycontinuously tapered from said first end to said second end withexternal threads defined on said outer surface and extendingsubstantially entirely along said length of said body.
 2. The fusiondevice according to claim 1, wherein said body is formed of a porousbiocompatible material to permit bone tissue ingrowth into the device.3. The fusion device according to claim 2 wherein said material is acomposite comprising an open-celled substrate having interconnectedporosity, said substrate infiltrated with a metal.
 4. The fusion deviceaccording to claim 3 wherein said substrate is a carbonaceous material.5. The fusion device according to claim 4 wherein said substrate is acarbon foam.
 6. The fusion device according to claim 3 wherein saidmetal includes a group VB metal.
 7. The fusion device according to claim6 wherein said metal is tantalum.